1. Field of the Invention
This invention relates generally to transition joints and more particularly, but not by way of limitation, to fixed-bottom lower transition joints for suspended pipe risers.
2. Description of the Prior Art (Prior Art Statement)
The following statement is intended to be a Prior Art Statement in compliance with the guidance and requirements of 37 C.F.R. 1.56, 1.97 and 1.98.
U.S. Pat. No. 3,976,021, issued to Blenkarn et al., shows at FIG. 10, a riser having a transition joint with a straight taper between the upper and lower surfaces of the joint. That transition joint is not fixed at either its upper or lower surface. Blenkarn et al. does not disclose a curvilinear taper or an optimal design for such a taper.
U.S. Pat. No. 3,605,413, issued to Morgan, discloses a riser having a rigidity varying lower portion which interconnects with an upper portion. The lower or base portion is disclosed to be made of steel and to have a non-uniform rigidity or section modulus wherein the maximum is at the foot of the base portion which connects to the seafloor structure, and wherein the minimum is at the top of the base portion which attaches to the upper portion.
To meet such criteria, the Morgan patent indicates that the base structure comprises a plurality of segments with each segment having a different outer diameter and wall thickness relative to every other segment. Although each segment has a different outer diameter, each has the same inner diameter. Each of these sections is interconnected so that the lowermost section has the largest diameter and each successively higher portion has a successively smaller outer diameter. Also, at the point of interconnection of each section there is a taper which compensates for the different outer diameters of the connected segments. It is disclosed in the patent that such tapering could extend along an entire segment.
In addition to the varying diameter segments, the base portion comprises rigidity transition structures which prevent abrupt changes in the radius of curvature and act as stress transfer members between the upper portion of the riser and the upper sections of the base portion of the riser.
Although the Morgan patent does indicate a transition joint comprising elements having different outer diameters, it fails to indicate a joint which has an outer surface which is continuously tapered from the top to the foot of the joint. Furthermore, the Morgan patent fails to disclose an optimally designed transition joint which has a nearly constant resultant stress along the length of its structure.
Other references which Applicant has knowledge of and which may be of relevance include U.S. Pat. No. 3,794,849 issued to Perry et al. which discloses a neutral buoyancy conductor connecting a floating power plant to stationary conductors which then connect the power plant to the shore. The neutral buoyancy conductor is indicated to have constant inner and outer diameters and to bend as a catenary to distribute stress resulting from various loads. The Perry et al. patent also discloses in its drawings vertical structures having continuously varying thicknesses from top to bottom. The specification indicates that these are poured concrete seawalls erected to form channels, but does not further define them.
As with the Morgan patent, the Perry et al. patent fails to show a transition joint which has a continuously varying outer diameter from top to bottom which is optimally shaped to have nearly constant resultant stress along the length of the joint.
Another patent of interest is U.S. Pat. No. 3,559,410 issued to Blenkarn et al. which discloses ring-type stress relief members. However, this patent fails to disclose a longitudinally extending, continuously curvilinearly varying outer diameter transition joint which has nearly constant resultant stress along the length of the structure.
Still another patent known to Applicant is U.S. Pat. No. 3,512,811 issued to Bardgette et al. which discloses a jacket-to-pile connector which has a partially varying thickness wall attached between a jacket leg and a pile to transfer horizontal loads therebetween. This patent, however, fails to indicate a longitudinally extending transition joint having a constant inner diameter, but a curvilinearly varying outer diameter and further having a nearly constant resultant stress along the length of the structure.
Finally, U.S. Pat. No. 1,706,246 issued to Miller discloses in its drawings vertical structures having a continuously varying or tapered outer surface. These vertical structures are walls which have linearly varying thicknesses from top to bottom. However, this patent fails to disclose optimum design criteria or any advantages for having the walls so tapered. Furthermore, this patent fails to disclose a transition joint having such a tapered contour.
As shown by the above-mentioned disclosures, there is a need for a transition joint which, in particular, joins a seafloor structure to a surface structure. There is also the need for such a joint to exhibit a size and strength which can resist the varying loads applied to it and yet to have an optimum design for economy of material and for ease of manufacture.
As indicated above, however, the prior references known and cited by applicant fail to meet the needs because they fail to disclose an optimally designed transition joint which can be particularly used in oil and gas production systems to connect a seafloor structure to a surface structure. In light of the failure of the prior references applicant believes that no previously disclosed device which is known to him indicates, either singly or in combination, the present invention.